This invention relates to a cable fairing and more particularly to a sectioned fairing. Underwater marine systems, which use a cable either as a tether or for towing, benefit from the utilization of cable fairings. Cable fairings are streamlined shapes which attach to underwater cable to reduce fluid drag force and eliminate cable strumming. They allow towed systems to attain significantly deeper depths with a particular length of cable and towing speed. They are a virtual necessity for any high-speed towing applications.
The general design criteria for cable fairings which minimize fluid drag forces are well established: streamlined shape, smooth surface, and a minimum number of surface discontinuities, such as gaps or slots. Practical applications for cable fairing impose additional design requirements. These include: durability, stability during tow, and resistance to the production of lateral forces or kiting. Existing fairing designs meet only some of the above criteria. Designs which are durable and low kiting tend to have high drag and poor performance. Low drag designs have been particularly susceptible to kiting.
Fairing durability and stability result from proper selection of materials and proper geometry. These requirements, by themselves, have been successfully met in numerous existing designs.
Kiting has remained a persistent problem, however, particularly in fairings which have been designed for low drag. This has caused tradeoffs in prior-art fairing designs to provide durability and free motion over cable sheaves. Cable kiting is caused by asymmetric fairing shapes, and/or non-alignment of fairings to the local flow, both which produce forces perpendicular to the flow of stream.
Asymmetric fairings act like cambered wings, and are capable of significant moment about the cable axis and concurrently large lift. Asymmetry may result from poor manufacturing tolerances or from cusping of the fairing trailing edge as the fairing reacts to cable bending and strains the tail section. To allow cables to bend freely without deforming the fairings, the fairings divided into segments along the cable axis. Additionally, much attention has been paid of the fairing nose (the region which surrounds or connects to the tow cable). Fairing designs in the U.S. Navy fleet service employ a rugged stamped steel nose which is curved along the cable axis to match the radius of the winch drum. This prior art design used with sheaves having a compliant supporting liner provides adequate clearance to allow cable bending without deforming the fairing segment. However, the curved (scalloped) leading edge and large gaps between fairing sections greatly increase the drag.
An alternate approach has been to manufacture fairing segments with a flexible nose material such as polyurethane which flexes as the cable bends as described in U.S. Pat. No. 3,611,976. The nose is attached to a rigid tail section which maintains the fairing shape. This approach results in a clean nose shape with virtually no gaps between sections, and therefore, little drag. Unfortunately, the flexible nose material cold-flows and creeps into the interstices of the cable armor and creates resistance to the fairing pivoting freely about the cable to the flow direction. This results in a small local attack angle to the fluid which creates large lateral forces and poor performance (kiting).
The pivoting resistance of the prior art flexible nose fairing is caused by high frictional resistance of the nose material against armored tow cable, and an inability to maintain the necessary clearance in the cable hole. Some flexible fairings have been designed to compress at the end of the nose to reduce the size of drag producing gaps between fairing segments. Additionally, the nose material has a tendency to creep between the armored cable strands after long term storage under tension, further increasing pivoting resistance.
It is therefore an object of this invention to provide a fairing segment which is flexible to conform to cable bending while being resistant to creep into the cable armor thereby allowing the fairing to rotate freely about the cable. It is a further object to provide a fairing which is non-kiting and which has low drag. It is further object of this invention that the fairing segment is also durable and stable. It is a further object to provide a fairing segment which is flexible to bend as the cable bends when towing or as it goes over a sheave and thus eliminate stress concentration in the cable at ends of the fairing segment. It is a further object of this invention to provide a fairing segment which may be molded as a unit which includes the cable hole flexible steel liner with the body of the fairing segment. It is a feature of the invention that the fairing segment surface is smooth and without gaps and seams from the nose to the tail. It is a further feature that substantially the entire interior surface of the fairing segment cable hole in the fairing segment is a flexible yet cold-flow resistant surface. It is, therefore, a feature of this invention that the cale hole in the nose of the fairing segment is lined with a flexible spring steel to prevent the nose material from cold flowing into the interstices of the cable armor. It is another feature that a fairing segment can be removed individually for replacement or repair by simple hand tools. It is a further feature of the invention that the leading edge of the nose of a fairing segment is chamfered to minimize longitudinal compression of the nose as the fairing is carried over a cable sheave.